1. Field Of The Invention
This invention relates generally to evaporative humidifiers, and specifically to humidifier pads used in evaporative humidifiers.
2. Description Of The Related Art
Evaporative humidifiers humidify air which is passed through paths formed in a wetted humidifier pad element. A flow of air is directed into the wetted pad to cause evaporation of the water absorbed by the pad. The evaporated water is carried off in the air passing through the pad element.
One form of humidifier pad commonly used currently is made of a plurality of superposed sheets or layers of perforated material, preferably adhesively bonded together. Typically, one form comprises sheets of absorbent paper. Another form comprises coated aluminum. The perforations can be conveniently provided by a conventional"slitting and expanding" process well-known in the trade. This process creates a plurality of generally diamond shaped openings.
The slit and expanded humidifier pads comprising layers of coated aluminum are advantageous for their strength and stability. However, the aluminum layers must be coated with an absorbent coating and are useful only in humidifiers which feed water to the pad from the top. The disadvantages of these all-metal pads is the high cost of aluminum and the associated coating process.
Evaporative pads comprising slit and expanded paper layers, with or without an outer frame or a supporting layer of slit and expanded aluminum, are significantly lower in cost than the all aluminum type pads. While all-paper pads do not have the same useful life span as the all aluminum pads, their useful life relative to cost has made them a practical and economic choice in many applications. Therefore, the evaporative pads comprising absorbent paper layers represent a highly successful market, particularly in the residential room humidifier units.
Water must be continuously supplied to the pad for evaporation to occur at a consistent, predictable rate. Water must be transported to the areas of the humidifier pad from which the water is evaporated: the regions surrounding the openings in the tortuous airflow paths through the layers. In the case of the paper type pads, this occurs in one of two ways. In a wicking humidifier system, which has a reservoir in which the lower portion of an absorbent, paper pad is disposed, water is absorbed and wicks up by capillary action through the fibrous matrix of the paper layers to moisten the entire pad. Alternatively, in a downflow humidifier system, water flows downwardly from the top of the humidifier pad, and both gravity and capillary action transport the replacement water uniformly throughout the pad. In both systems, water is present at, and resupplied to, the paper media surrounding the openings in the layers of the pad so that impinging air can entrain and carry off the water vapor evaporated from the pad.
As is well known and conventional in the art, wicking up type humidifiers are preferred for portable, residential room type humidifier units because no piping connection to an external water supply is necessary. An example of a wick-up humidifier is shown schematically in FIG. 4. This type of unit relies upon a reservoir 2 of water which must be replenished from time to time and an evaporative wicking pad element 4 having its lower portion submerged in the water reservoir 2. A larger upper portion of pad element 4 is exposed to a flow of air driven by a fan 6. Typically, such units are designed for a given output capacity measured by the volume of water per unit time. Units having different output capacities are made to offer the user with appropriate choices relative to the particular output needed for a given application.
The output capacity of a given unit depends upon several parameters, including the temperature and humidity level of the incoming air, the rate of air flow delivered to the evaporative pad, and the size, composition and effective surface area of the evaporative pad.
The width of the pad is limited by size and cost limitations in designing such residential room-sized units. In addition, there is a proportional increase in the size of the motor and fan unit necessary to supply the necessary flow of air to aid evaporation and carry off the water vapor.
Since the supply of water to the exposed portion of the pad is derived via capillary action from the lower portion of the pad submerged in the reservoir, it has been found that 7 or 8 inches above the water level, the rate of evaporation under typical use conditions is sufficiently greater than the rate water can be effectively transported to these upper portions of the pad. Therefore, it becomes impractical to increase output capacity by merely increasing the height of the pad exposed above the water level beyond about 10 inches or so for the typical portable residential room humidifier.
As noted earlier herein, these wick-up type humidifier pads rely upon capillary action to provide a continuous supply of water to maintain the area of the portion of the pad exposed to the air flow in a wetted condition. Therefore, the conventional pad for this type of unit employs only layers or sheets of highly absorbent paper having a high rate of capillary rise. The output capacity is then varied by the dimensions of the pad, including its thickness. In portable residential room type humidifiers, capacity is primarily varied by the numbers of layers of wicking paper in the pad.
Although the practical height and width of pads are limited as noted above, the thickness of a pad can vary substantially. Room size humidifier units, using the highly absorbent wicking paper layers have successfully provided a very sufficient range of capacity using relatively small dimensioned humidifier pads varying in thickness from about 1 to 3 inches using a range of about 7 to 28 layers of wicking paper. The majority of such units usually employ 14 or more layers of wicking paper.
Generally speaking, suitable highly absorbent papers which have high rates of capillary rise and sufficient strength necessary to withstand the slitting and expanding process are significantly more expensive than other forms of Kraft paper having relatively low capillary rise properties.
In view of the highly competitive nature of the relevant market, there is always a need to reduce the cost of this type of humidifier pad without significant sacrifice of the necessary functional features, including the capacity or level of output of any given design for a humidifier unit.